One of the US's top operators of theme parks, Six Flags, declared itself bankrupt over the weekend after struggling to refinance $2.4bn (£1.5bn) of debt, posing the prospect of a financial roller-coaster ride for major shareholders including the Micro­soft billionaire Bill Gates.

Six Flags operates 20 parks in the US, Canada and Mexico including Los Angeles's Magic Mountain, Chicago's Great America and Montreal's La Ronde. The company filed for chapter 11 protection with a repayment of $287m to creditors due in August.

Chief executive Mark Shapiro was anxious to reassure visitors that the company's parks will stay open over the peak summer season. "This isn't a liquidation," he said. "The summer is here – all the parks are open at full throttle."

He said Six Flags' debt problems date back a decade to a period when the company expanded aggressively, buying parks as far afield as Belgium, Germany and France and investing heavily in state-of-the-art rides.

"If the markets were better, maybe you could refinance the debt," Shapiro told CNBC television. "But that's just kicking a can down the road, it's not solving the problem."

Six Flags' shareholders could be wiped out or, at best, heavily diluted as the company pushes a bankruptcy judge to force creditors to swap their loans for stock. This would be a blow to Gates, whose private investment vehicle, Cascade Investments, owns an 11% stake.

The theme park operator's other shareholders include the Washington Redskins' owner, Daniel Snyder, who has 6% and who chairs the Six Flags board. The hedge fund Renaissance Technologies, run by billionaire stockpicker Jim Simons, has a 5.5% chunk.

Six Flags had hoped to capitalise on a trend towards cash-strapped Americans spending "staycations" close to home rather than taking holidays abroad. The company generated revenue of $1.02bn in the year to March but after interest repayments it was $112m in the red.

In an open letter to its 2,040 employees, the company said it would continue to pay wages in full. "This restructuring process is strictly a 'back of the house' effort to address and ensure the longstanding financial stability of Six Flags."

Six Flags is the latest in a long line of high-profile US bankruptcies inclu­ding the carmakers General Motors and Chrysler, the shopping centre owner General Growth and retailers such as Circuit City and ­Linens 'n Things.